The synergistic effect of gemini surfactants (GS) 1,2-ethane bis(dimethyl alkyl ammonium bromide) (C_mH_2m+1) (m-2-m, m = 10 and 12) and n-butanol (C₄OH) on the corrosion of mild steel in 1 M HCl solution has been investigated using gravimetric method, electrochemical methods (PDP and EIS) and scanning electron microscopy (SEM). Experimental outcome suggests that the inhibition efficiency (η) of m-2-m GS is considerably improved in the presence of butanol. The values of synergism parameter (S_θ) are greater than unity, indicating high η caused by the addition of C₄OH to m-2-m GS is due to a co-operative adsorption. Polarization studies showed that the m-2-m GS in the presence or absence of C₄OH are mixed-type inhibitors. Analysis of variance (ANOVA) reveals that the difference existing between inhibition efficiencies obtained by PDP, EIS and gravimetric techniques is statistically insignificant. The adsorption of studied inhibitors on mild steel surface in 1 M HCl solution followed the Langmuir adsorption isotherm. Surface morphology was studied using SEM and AFM. Some quantum chemical parameters and the Mulliken charge densities for m-2-m GS were calculated by the ORCA programme to provide further insight into the inhibition mechanism of the corrosion process.